1. Field of the Invention
The present invention relates to an apparatus and method for interferometrically measuring the absolute distance between a plano test surface and a plano reference surface which are in close proximity to each other. More particularly, the invention relates to apparatus which rapidly and accurately measures said absolute distance and which requires no physical contact with the surface under test.
2. The Prior Art
Interferometers are generally known for determining distances and the topography of a test surface; see, for example, C. Zanoni, "Interferometry," The Optical Industry and Systems Purchasing Directory, Book 2, pp. E-80--E-82 (1983). Interferometry relies ultimately on the measurement of phase. In traditional interferometry, the measurement of phase is derived from the geometry of the fringe pattern.
Phase measuring interferometry ascertains the phase at each point in the interference pattern by measuring the corresponding intensity variation as the overall phase is modulated. The phase must be modulated by 2.pi. in a precisely known way so as not to introduce errors in the measured intensities, see for example, J. H. Bruning, et al., "Digital Wavefront Measuring Interferometer for Testing Optical Surfaces and Lenses," Applied Optics, vol. 13, pp. 2693-2703 (1974); Gallagher, et al., U.S. Pat. No. 3,694,088 issued Sept. 26, 1972; N. Balasubramanian, U.S. Pat. No. 4,225,240 issued Sept. 30, 1980; M. Schaham, "Precision optical wavefront measurement," Proceedings SPIE, vol. 306, pp. 183-191 (1981); and H. Z. Hu, "Polarization heterodyne interferometry using a simple rotating analyzer. 1: Theory and error analysis," Applied Optics, vol. 22, pp. 2052-2056 (1983).
In prior-art phase measuring interferometers, the phase is modulated by a known amount by one of the following means: (1) mechanically moving an optical element of the interferometer with a piezoelectric transducer, (2) rotating a phase retardation plate in the interferometer, (3) use of either an acousto-optic, electro-optic, or similar device in the interferometer, and (4) variation of the incident angle, see for example, R. C. Moore, U.S. Pat. No. 4,325,637 issued Apr. 20, 1982.
In my copending application, Ser. No. 515393, "Interferometric Wavefront Measurement," assigned to the assignee of this application, an improved phase modulation apparatus and method is disclosed; however, it cannot be used in either an equal path interferometer or an interferometer with a cavity length equal to or less than the optical length of the diode laser.
While the prior-art fringe pattern and phase measuring interferometers are useful for many applications, they cannot be used for some measurements. For example, in the computer mass storage system industry it is required to measure the flying height of a magnetic head assembly on a rapidly rotating disk in order to verify the performance of the magnetic head assembly. The flying height is the distance between the magnetic head assembly's rails, also referred to as air bearing surfaces, and the surface of the rotating disk, see M. F. Garnier, et al., U.S. Pat. No. 3,855,625 issued Dec. 17, 1974. The flying height results from the aerodynamic effects produced by the disk's rotation and ranges from 0.1 to 0.25 micrometers. In addition to the flying height, it is desirable to measure the topography as well as the angular orientation of the rails, or portions thereof, in order to assess the compliance of these parameters to the design specifications. It is desirable to measure the aforementioned parameters quickly and automatically with minimum operator intervention. For this application, the distance to be measured is nominally less than one-half of a wavelength of visible light, and the absolute distance must be measured.
Prior-art apparatus and methods for measuring the flying height of a magnetic head assembly have included: (a) visual assessment of the color bands produced by white light interferometry, (b) multiple wavelength interferometry, and (c) capacitive-type sensors. The prior-art optical techniques have generally measured the flying height of a magnetic head assembly using a rapidly rotating glass disk, one surface of which is a reference surface of an interferometer. White light interferometry suffers from a number of limitations; first, as the flying height gets below one-half a wavelength of light, i.e., approximately 0.3 micrometers, only limited and ambiguous information is available; second, it does not lend itself to automated operation for high through-put production testing. A single manufacturer typically produces 200,000 to 500,000 magnetic head assemblies per month. Similarly, the multiple wavelength interferometry technique suffers from the same limitations. The capacitive sensor approach is suitable for some laboratory testing but requires that a capacitive transducer be added to the magnetic head assembly to be tested. For production testing this is neither practical nor cost effective. Furthermore, all of the aforementioned techniques provide poor spatial data sampling.
While prior-art fringe pattern and phase measuring interferometers are useful for many applications, they cannot measure the absolute distance between a plano test surface and a plano reference surface which are in close proximity to each other.